Rectilinear actuator and potentiometer



Aug. 8, 96. w. E. MOWER ETAL 3,335,385

RECTILINEAR ACTUATOR AND POTENTIOMETER Filed July 29. 1964 Q 4Sheets-Sheet 1 [Li V FIG I INVENTORS WILFORD E. MOWER JAMES H. PEPPERDOUGLAS H. SMITH ATTORNEY g- 8, 1967 w. E. MOWER ETAL 3,335,385

RECTILINEAR ACTUATOR AND POTENTIOMETER Filed July 29, 1964 4Sheets-Sheet 2 INVENTORS WILFORD E. MOWER JAMES H. PEPPER DOUGLAS H.SMITH FIG. 6

AYTORNEY g- 1967 w. E. MOWER ETAL 3,335,385

RECTILINEAR ACTUATOR AND POTENTIOMETEH Filed July 29. 1964 4Sheets-Sheet 3 FIG. 4

FIG 5 INVENTORS WILFORD E. MOWER JAMES H. PEPPER DOUGLAS H. SMITH AORNEY Aug. 8, 1967 w. E. MOWER ETAL 3,335,335

RECTILINEAR ACTUATOR AND POTENTIOMETER Filed July 29. 1964 4Sheets-Sheet 4 (D r- Q I Q \J 6 I I I v I (11 -J A V 2 b I O3 (9' IIINVENTO S WILFORD E. MOWER JAMES H. PEPPER DOUGLAS H. SMITH ATTORNEYUnited States Patent 3,335,385 RECTILINEAR ACTUATOR AND POTENTIOMETERWilford E. Mower, Concord, James H. Pepper, Orinda, and Douglas H.Smith, Albany, Calif., assignors to Beckman Instruments, Inc., acorporation of California Filed July 29, 1964, Ser. No. 385,839

7 Claims. (Cl. 338116) This invention generally relates to rectilinearpotentiometers and more particularly to a unitary rectilinear actuatorand rectilinear potentiometer.

In the field of analog computer applications it has been a generalpractice to utilize servo-set coefiicient potentiometers for numerousapplications. These generally comprise copper mandrel potentiometers orpotentiometers of other types driven by a small servo motor. Suchdevices have an average volume of approximately 50 in. and an averageend-to-end setting time of approximately 2 seconds. Since in many analogcomputers large numbers of such devices are utilized the size of thecomponents 'become extremely important as does their speed of operationin the overall size and speed of operation of the computer.

It is, therefore, an object of the present invention to provide arectilinear actuator and rectilinear potentiometer combination which maybe utilized as a servo-set coeflicient potentiometer in analogapplications, that is greatly reduced in size and which has an extremelyfast end-to-end setting time when compared with the present dayservo-set coefficient otentiometers.

It is another principal object of the present invention to provide arectilinear actuator and rectilinear potentiometer combination which maybe readily adapted to a multiplicity of functions without modification,which is simple of construction, small in size, rapid in response, lowin cost and is highly accurate.

Still another object is to provide a rectilinear actuator andrectilinear potentiometer combination which may readily be utilized as aservo-set coefiicient potentiometer, a track and store device, a servomultiplier or a servo function generator without significantmodification or adaptation.

A further object is the provision of a rectilinear actuator andrectilinear potentiometer in which an armature carrying a slider may beaccurately and rapidly positioned at any point along a resistanceelement.

Another object is to provide a rectilinear actuator and rectilinearpotentiometer combination which may be utilized in a variety ofapplications without modification utilizing an armature carrying a wiperalong a resistance element which may be rapidly positioned with highaccuracy and in which there is no restoring force thus allowing thedevice to be utilized as a storage element.

Still another object is to provide a rectilinear actuator andrectilinear potentiometer combination utilizing an armature-wiperassembly having a low mass such that the wiper may be rapidly andaccurately positioned along a resistance element and in which thefrictional forces may readily be made to exceed gravitational forcessuch that the device may be made essentially independent of spacialorientation.

Other objects and many attendant advantages of this invention and manynovel arrangements and combinations of parts will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings in which like reference numerals indicate likeparts throughout the several views thereof and wherein:

FIG. 1 is a sectional view of one preferred embodiment taken along line11 of FIG. 2;

FIG. 2 is a sectional view of the embodiment taken along line 22 of F G.1;

FIG. 3 is an isometric view of the free running armature of theembodiment of FIGS. 1 and 2;

FIG. 4 is a sectional view of a second preferred embodiment taken alongline 4 4 of FIG. 5;

FIG. 5 is a sectional view of the embodiment taken along line 55 of FIG.4;

FIG. 6 is an isometric view of the free running armature of embodimentof FIGS. 4 and 5;

FIG. 7 is a sectional view of a third preferred embodiment;

FIG. 8 is an electrical schematic diagram of a coefiicient potentiometerconstructed after the teachings of this invention; and

FIG. 9 is an electrical schematic diagram of a single quadrantmultiplier constructed after the teaching of this invention. 7 Referringnow to FIGS. 1, 2 and 3 there is illustrated one preferred embodiment ofa rectilinear actuator and rectilinear potentiometer combinationconstructed after the teachings of this invention. The unit comprises ahousing having a main body portion 12 and first and second end members13 and 14. End member 13 has an aperture 16 therein and end member 14has a boss 17 formed on the internal surface, the boss having a blindaperture 18 in axial alignment with aperture 16 of end member 13.Supported within apertures 16 and 18 is a permanent magnet 20 which mayreadily comprise an Alnico slug in the shape of a truncated cone havingits end portions cylindrically cut for receipt in apertures 16 and 18.

A plastic guide member 21 is formed about the slug 20 in such a manneras to form a cylindrical guide. The main body portion 12 and end member13 may conveniently be made of soft iron to form a highly permeable fluxpath for the flux of permanent magnet 20. End member 14 may beconveniently constructed of phenol. It is apparent that an air gap isformed between the guide member 21 and the soft iron housing in the fluxpath between the permanent magnet and the housing.

Supported by guide member 21 is a free running armature assemblygenerally comprising a cylindrical insulating sleeve 25 having aprotrusion 26 for carrying one or more electrically conductive,resilient fingers which may form the sliders or wipers. In theparticular embodiment illustrated two sliders 27 and 28 are illustrated.Sliders 27 and 28 may be afiixed to the protrusion 26 in any suitablemanner and are electrically connected. Embedded within sleeve 25 is anarmature coil 32 having its ends thereof respectively brought outthrough protrusions 33 and 34 and terminating in flexible braids 36 and37 thus forming a suitable means of energizing coil 32.

A resistance element 39 and an electrically conductive strip 40 aresupported in space relation by insulating member 42, resistance element39 and conductive strip 40 being in contact respectively with resilientsliders 27 and 28.

Conductive strip 40 may be brought to an external terminal, not shown,and provides a convenient means of external connection to slider 27.

The sliders and protrusions 33 and 34 serve the multiple functions ofproviding electrical contacts to the resistance and conductive elementsand a means of support for the lead wires of coil 32 and as axial guidesfor the free running armature assembly.

The magnetic structure establishes a strong radial magnetic field in theair gap between the permanent magnet and the soft iron housing. Currentflow through the armature coil creates a magnetic field which reactswith the radial field established by the permanent magnet therebycreating a force for moving the armature assembly axially along theguide 21 in a direction which depends upon the direction of current flowin the armature coil. It is apparent that; by reversing the direction ofcurrent flow through the armature coil the direction of movement of thearmature asesmbly may be reversed. If the force developed by thereacting magnetic fields is sufficient to overcome the frictional forcesof the system, the armature assembly will move along the surface of theguide thereby moving wipers 27 and 28 along their respective elements.By forming the armature assembly of a suitable insulating material, suchas the material sold under the trademark Fiberglas, the mass of thearmature may be made low and the frictional forces can be made to exceedthe forces due to gravity thereby making the device essentiallyindependent of spacial orientation. Since there is no restoring forcethe device may also be utilized as a storage element. When theresistance element is connected in a feedback circuit-the armatureassembly can be rapidly positioned with a precision which is a functionof the characteristics of the resistance element and the servo loopgain.

Referring now to a second embodiment constructed after the teachings ofthis invention which incorporates a plurality of resistance elementswhich may be utilized in various configurations. The embodiment differsmainly in the free running armature assembly, best illustrated in FIG.6, which again may comprise a low mass sleeve 46 made of an insulatingmaterial. The armature assembly has a first pair of diametricallyopposite protrusions 47 and 48 located at one end of the sleeve and asecond pair of diametrically opposite protrusions 51 and 52 located atthe other end thereof and rotated 90 with respect to the first pair.

The resilient, electrically conductive contacts or sliders 53 and 54respectively secured to protrusions 48 and 52 may serve as the terminalsof armature coil 56 embedded in armature assembly 46. Supported adjacentand in electrical contact with sliders 53 and 54 are electricallyconductive strips 58 and 59 respectively supported within the main bodyportion of the housing by any suitable means such as, for example,insulating members 61 and 62 respectively. It is to be understood thatthe electrically conductive strips 58 and 59 are brought to the exteriorof the housing by any suitable means, not shown, for connection toexternal circuitry. Protrusion 47 of the armature assembly may, ifdesired, carry a pair of electrically connected sliders 63 and 64 andrespectively engaging resistance element 66 and conductive strip 67spacially supported upon an insulating member 69 secured within the mainbody portion of the housing. Protrusion 51 carries a slider 71 engagingresistance element 72 supported upon an insulator 73. Protrusion 51 may,if desired, include a second slider and a conductive strip to providefor the external connection of slider 71. On the other hand, slider 71may be brought to the external surface through an insulated, flexiblebraid similar to the coil leads in FIG. 3.

It is apparent that by bringing the coil leads to a slider and aconductive slide wire the necessity of the flexible braids of theembodiment of FIGS. 1, 2 and 3 is avoided.

Referring now to FIG. 7 there is illustrated a third embodiment of arectilinear actuator and rectilinear potentiometer combinationconstructed after the teachings of this invention and generallycomprises a main body housing 77 preferably constructed of soft iron andhaving an FIGS. 4, and 6, there is illustrated.

aperture extending through the longitudinal axis thereof. A permanentmagnet slug 78 in the form of a solid cylinder and preferably of Alnicois supported within the housing by an end plug 79 preferably of softiron and an intermediate plug member 80 having a blind aperture thereinfor receiving the slug 78. Intermediate plug 80 is supportedsymmetrically about the axis of the device by a screw engaging threads81 therein and extending through the end plug 83 which may preferably beconstructed of a non-permeable material such as brass. A cylindricalinsulating member 84 is supported by end plug 83 and is utilized inmounting the resistance and pick off elements about the axis of thedevice, only two of which are illustrated at 85 and 86.

A non-ferrous guide member 87 is supported about the permanent magnetslug 78 between end plug 79 and intermediate plug 80 and is utilized asa guide for elongated sleeve member 89. As in the other embodiments, thesleeve member 89 forms a free running armature assembly composed of anon-ferrous insulating member carrying on its internal surface aplurality of wipers, two of which are shown at 91 and 92. Embedded inthe surface of sleeve member 89 is an armature coil 93. It is to beunderstood that the embodiment of FIG. 7 may utilize any number ofresistance elements and pick offs in the same manner as the embodimentsof FIGS. 1-6.

Referring now to FIG. 8 there is illustrated an electrical schematicdiagram of the device when utilized as a coefficient potentiometer. Inmost applications, the low potential end of the actuator coil and thepotentiometer or potentiometers may be grounded through the iron housingof the device. Therefore, in the instant embodiment only three wiresneed be brought to the external housing and only one of these, thecoeflicient potentiometer output lead, need be shielded.

As illustrated in FIG. 8 one end of potentiometer 101 and one end ofarmature actuating coil 102 are connected to the iron housing at 103which may be connected to a common point of potential, usually ground.The other end of the resistance element 101 is connected to an externalterminal 105 which is adapted to be connected to any source of suitablepotential. Slider 107 interconnects resistance element 101 andconductive slide wire 109 which is connected to a second externalterminal 110. The nongrounded end of actuator coil 102 is brought toexternal terminal 111 which forms an input terminal to the actuatingcoil. To form a null balance servo-loop, differential amplifier 113 hasone of its input terminals connected to terminal 115 adapted to receivean armature actuating signal and its other input terminal connectedthrough lead 116 to output terminal of the coefiicient potentiometer.The output of the differential amplifier is connected to terminal 111 ofthe actuator coil 102.

In operation, a suitable potential is applied across resistance element101 and the output potential developed between output terminal 110 andground is a function of the position of slider 107. A signal having anamplitude proportional to the desired setting of the coefficientpotentiometer is applied to input terminal 115. Differential amplifier113- compares the input signal with the feedback signal from outputterminal 110 and applies to actuator coil 102 a signal which isproportional to the difference between the input signal and the feedbacksignal and which has a polarity dependent upon which of these signals isgreater. Current flowing through coil 102 develops a magnetic fieldwhich reacts with the radial field produced in the air gap between thepermanent magnet and the soft iron housing to develop a force whichmoves the armature assembly in one direction or the other depending uponthe polarity of the input signal. The armature continues to move untilthe feedback signal is of such amplitude as to null the input signal. Atnull condition, the driving force is removed and the armature assemblyremains where it is stopped since there is no restoring force. It isapparent that any of the embodiments previously set forth herein, sinceeach contains at least one potentiometer element, may be utilized as acoefficient potentiometer in the circuit illustrated in FIG. 8.

It is apparent that with slight modification and when combined with aservo amplifier the coefficient potentiometer of FIG. 8 may be convertedto a track and store device. By introducing between the output ofamplifier 113 and terminal 111 a suitable switching device 117,illustrated in phantom in FIG. 8, it is apparent that the device willtrack the input signal applied at 115 and will store the value of thatsignal which exists at the time a store command is received by switch117 which operates to interrupt the input to coil 102 since, ashereinbefore pointed out, there is no restoring force and the freerunning armature will remain in the position it had when the inputsignal thereto was interrupted. If i reference signals are appliedacross potentiometer 101, signals of either polarity at input terminal115 may be tracked and stored. In this type of circuit, it would bedesirable to have a grounded center tap on resistance element 101.

Referring now to FIG. 9 there is illustrated how an embodimentconstructed after the teachings of this invention incorporating at leasttwo resistance elements may be connected as a single quadrantmultiplier. One end of potentiometers 121 and 122 and one end ofarmature actuating coil 123 are connected to the iron housing at 124.The other end of resistance element 121 is connected to an externalterminal 126 which is adapted to be connected to any source of suitablepotential and slider 127 interconnects resistance element 121 andconductive slide wire 128, slide wire 128 being connected to a secondexternal terminal 130. In a similar manner resistance element 122 hasits non-grounded end connected to an external terminal 132 andconductive slider 133 interconnects resistance element 122 and slidewire 134 which is connected to external terminal 135.

The non-grounded end of actuator coil 123 is brought to an externalterminal 137 in any suitable manner as hereinbefore described. Inputterminal 139 is adapted to receive a signal which is a function of x andis connected through a comparator 140 and operational amplifier 141 toterminal 137. Slider 127 is connected through slide Wire 128 and outputterminal 130 to comparator 140. The operation of the closed loopfeedback servo system of the embodiment of FIG. 9 is similar to that ofFIG. 8. If a fixed reference signal F is applied to terminal 126 and asignal which is a function of x applied to terminal 139, the freerunning armature assembly will move until the feedback signal issubstantially equal to the input signal. The position of slider 127, andhence the position of slider 133, is then proportional to Fx. If asignal that is a function of y is applied to input terminal 132, theoutput signal at terminal 135 is kxy, where k is a constant.

It is obvious that the embodiment of FIG. 9 may also be utilized as aservo function generator if the linear resistance element 121 isreplaced by a non-linear element having the desired function. 3

By utilizing a one inch resistance element the overall structure may bemade less than 1 in. by approximately 2 /2 inches long for a totalvolume of approximately 2.5 in. This compares with an average volume ofapproximately 50 in. for servo-set coefiicient potentiometers presentlyavailable. This represents a reduction in size by a factor of 20.End-to-end setting times of less than ms. may readily be achieved. Thiscompares with a setting time of approximately 2 seconds for conventionalservo-set coefiicient potentiometers makingthe device of the instantinvention approximately 200 times faster.

The particular type of resistance element utilized in the embodimentsconstructed after the teachings of this inventionforms no part of theinvention and Will depend only upon availability and the particularapplication. However, the utilization of cermetresistance elements hasbeen found advantageous.

The cermet potentiometer has an inherent band width of approximately 1me. which compares with approximately 6 kc. for present copper mandrelpotentiometers, an increase by a factor of 160. This increased bandwidth means that no phase compensation is required in high performanceequipment with the attendant elimination of load matching problems.Because of the small size of the over-all structure, short system wirelengths make it possible to exploit this improved band width in a largesystem. Further, the resolution of a cermet resistance element is, formost practical purposes, infinite. The accuracy of the setting of thecoefficient potentiometer is, therefore, limited only by factors such asservo loop gain, reference accuracy and stability, and closed loopsystem stability. No difiiculty has been found in achieving accuraciesof 1 part in 10,000 which is equivalent to present servo-set coefiicientpotentiometers. Temperature coefiicients of p.p.m./ C. have beenachieved in cermet resistance elements. This temperature stability isfurther inherently enhanced in a device constructed after the teachingsof the present invention since the soft iron housing in which theresistance element is mounted acts as a heat sink preventing largetemperature changes due to self heating.

It should be understood that while the armature assembly has beendisclosed as being of insulating material, in some applications a largeamount of damping may be desired. If this be the case, it is apparentthat the armature may be made of a conducting material such, forexample, as aluminum and the actuating coil and the sliders must thenobviously be insulated therefrom. It should also be understood thatvarious permanent magnet configurations may be utilized in order to'control the flux distribution in the air gap.

Many modifications and variations are possible in the embodiments, whichare illustrated by way of example only and not by way of limitation, inlight of the teachings of this invention without departing from thespirit and scope thereof which is defined by the appended claims.

What is claimed is:

1. In a rectilinear actuator and potentiometer combination formingtherebetween an elongated air gap, said magnetic means establishing afirst magnetic field within said air gap;

a free running armature supported within said air gap by said magneticmeans and including actuating gap of said magnetic means depending uponthe orientation of said magnetic fields;

at least one rectilinear resistance element supported within saidhousing and longitudinally disposed therein;

an electrically conductive slider means carried by said armature andengaging said resistance element; and

means for connecting said resistance element, said slider means and saidactuating means to external circuitry whereby said slider means may bepositioned along said resistance element in response to energization ofsaid actuating means.

2. In a rectilinear actuator and potentiometer combination magneticmeans comprising a housing including an elongated highly permeable bodyportion and an elongated magnet supported within said housing andforming therebetween an elongated air gap, said magnetic meansestablishing a first magnetic field within said air gap;

a free running armature supported within said air gap by said magneticmeans and movable along at least a substantial portion of the length ofsaid elongated body portion;

actuating means carried by said armature for establishing a secondmagnetic field reacting with said first magnetic field for driving saidarmature in one direction or the other along said elongated body portiondepending upon the orientation of said magnetic field;

at least one rectilinear resistance element supported within saidhousing and longitudinally disposed therein;

slider means carried by said armature and engaging said resistanceelement; and

means for connecting said resistance element, said slider means and saidactuating means to external circuitry whereby said slider means may bepositioned along said resistance element in response to energization ofsaid actuating means.

3. In a rectilinear actuator and potentiometer combination magneticmeans comprising a housing including an elongated highly permeable bodyportion and an elongated magnet supported within said housing andforming therebetween an elongated air gap, said magnetic meansestablishing a first magnetic field within said air gap;

a free running armature supported within said air gap by said magneticmeans and movable along at least a. substantial portion of the length ofsaid body portion;

coil means supported by said armature for establishing a second magneticfield within said air gap upon energization of said coil means, thereacting forces of said magnetic fields driving said armature in onedirection or the other depending upon the direction oi current flowWithin said coil;

at least one rectilinear resistance element supported within saidhousing and longitudinally disposed therein;

electrically conductive slider means carried by said armature andengaging said resistance element; and

means for connecting said resistance element, said slider means and saidcoil means to external circuitry whereby said slider means may bepositioned along said resistance element in response to energization ofsaid coil.

4. A rectilinear actuator and potentiometer combination comprising:

means and forming with said body portion an air a free running armaturesupported by said guide means,

for longitudinal motion within saidair gapg' actuating coil meanssupported by said armature for moving said armature in either directiondependent upon the direction of current flow within said coil means;

at least one rectilinear resistance element supported within saidhousing adjacent said armature;

electrically conductive slider means carried by said armature andengaging said resistance element; and

I means for connecting said resistance element, said slider means andsaid coil to external circuitry whereby said slider means may bepositioned along said resistance element in response to energization ofsaid actuating coil means.

5, A rectilinear actuator and potentiometer combination comprising:

elongated magnetic means supported within said housing and establishinga transverse magnetic field between said magnetic means and said bodyportion;

guide means longitudinally disposed on said magnetic means and formingwith said body portion an air a free running armature supported by saidguide means for longitudinal motion Within said air gap;

actuating coil means supported by said armature for moving said armaturein either direction dependent upon the direction of current flow withinsaid coil means;

at least one rectilinear resistance element supported within saidhousing adjacent said armature;

electrically conductive slider means carried by said armature andengaging said resistance element;

a pair of electrically conductive strip means supported by said housingadjacent said armature;

a pair of slider means carried by said armature and respectivelyconnected to opposite ends of said coil, respective ones of said pair ofslider means engaging said first and second electrically conductivestrip means; and

means for connecting said resistance element, said electricallyconductive slider means and said electrically conductive strips toexternal circuitry whereby said electrically conductive slider means maybe positioned along said resistance element in response to energizationof said actuating coil.

6. A rectilinear actuator and potentiometer combination comprising:

a housing including an elongated highly permeable body portion;

an elongated magnet supported within said housing and establishing aradial magnetic field therebetween;

guide means supported by said elongated magnet and forming with saidbody portion an air gap;

a free running armature supported by said guide means within said airgap;

actuating coil means connected to said armature for moving said armaturein either direction dependent upon the direction of current flow withinsaid coil means;

a pair of electrically conductive strip means supported within saidhousing adjacent said armature;

a pair of sliders carried by said armature and respectively connected toopposite ends of said coil, respective ones of said pair of slidersengaging respective ones of said pair of electrically conductive strips;

a plurality of rectilinear resistance elements supported within saidhousing adjacent said armature;

a like plurality of electrically conductive slider means carried by saidarmature, respective ones of said plurality of electrically conductiveslider means engaging respective resistance elements; and

means for connecting said resistance elements, said plurality ofelectrically conductive strip means and said plurality of electricallyconductive slider means to external circuitry whereby said plurality ofslider means may be positioned along their respective resistanceelements in response to energization of said actuating coil.

7. A rectilinear actuator and potentiometer combina- 5 tion comprising:

a housing including an elongated highly permeable body portion;

elongated magnetic means supported within said housing and establishinga transverse magnetic field between said magnetic means and said bodyportion;

guide means longitudinally disposed on said magnetic means and formingwith said body portion an air a free running armature supported by saidguide means for longitudinal motion within said air gap;

actuating coil means connected to said armature for moving said armaturein either direction dependent upon the direction of current flow withinsaid coil means;

means for connecting said coil means to external circuitry;

at least one elongated, rectilinear resistance element supported withinsaid housing adjacent said armature;

at least one electrically conductive strip means supported within saidhousing adjacent said armature;

electrically conductive slider means carried by said armature andengaging said resistance element and said electrically conductive stripmeans; and

means for connecting said resistance element and said electricallyconductive slider means to external circuitry whereby said slider meansmay be positioned along said resistance element in response toenergize,- tion of said actuating coil.

References Cited UNITED STATES PATENTS 15 RICHARD M. WOOD,

Johnston et al. 338-116 Gilbert 338-116 Flaming 335-148 Massa 335-222Davis 335-222 Massa 335-222 Massa 335-222 Layland 338-183 Layland338-183 Primary Examiner.

W. D. BROOKS, Assistant Examiner.

6. A RECTILINEAR ACTUATOR AND POTENTIOMETER COMBINATION COMPRISING: AHOUSING INCLUDING AN ELONGATED HIGHLY PERMEABLE BODY PORTION; ANELONGATED MAGNET SUPPORTED WITHIN SAID HOUSING AND ESTABLISHING A RADIALMAGNETIC FIELD THEREBETWEEN; GUIDE MEANS SUPPORTED BY SAID ELONGATEDMAGNET AND FORMING WITH SAID BODY PORTION AN AIR GAP; A FREE RUNNINGARMATURE SUPPORTED BY SAID GUIDE MEANS WITHIN SAID AIR GAP; ACTUATINGCOIL MEANS CONNECTED TO SAID ARMATURE FOR MOVING SAID ARMATURE IN EITHERDIRECTION DEPENDENT UPON THE DIRECTION OF CURRENT FLOW WITHIN SAID COILMEANS; A PAIR OF ELECTRICALLY CONDUCTIVE STRIP MEANS SUPPORTED WITHINSAID HOUSING ADJACENT SAID ARMATURE; A PAIR OF SLIDERS CARRIED BY SAIDARMATURE AND RESPECTIVELY CONNECTED TO OPPOSITE ENDS OF SAID COIL,RESPECTIVE ONES OF SAID PAIR OF SLIDERS ENGAGING RESPECTIVE ONES OF SAIDPAIR OF ELECTRICALLY CONDUCTIVE STRIPS;